J.B. Smeets (Jeroen)http://repub.eur.nl/ppl/5236/
List of Publicationsenhttp://repub.eur.nl/eur_signature.pnghttp://repub.eur.nl/
RePub, Erasmus University RepositoryCan illumination estimates provide the basis for color constancy?http://repub.eur.nl/pub/25294/
Tue, 24 Mar 2009 00:00:01 GMT<div>J.J.M. Granzier</div><div>E. Brenner</div><div>J.B. Smeets</div>
Objects hardly appear to change color when the spectral distribution of the illumination changes: a phenomenon known as color constancy. Color constancy could either be achieved by relying on properties that are insensitive to changes in the illumination (such as spatial color contrast) or by compensating for the estimated chromaticity of the illuminant. We examined whether subjects can judge the illuminant's color well enough to account for their own color constancy. We found that subjects were very poor at judging the color of a lamp from the light reflected by the scene it illuminated. They were much better at judging the color of a surface within the scene. We conclude that color constancy must be achieved by relying on relationships that are insensitive to the illumination rather than by explicitly judging the color of the illumination. Relapse and Stability of Surgically Assisted Rapid Maxillary Expansion: An Anatomic Biomechanical Studyhttp://repub.eur.nl/pub/25051/
Thu, 01 Jan 2009 00:00:01 GMT<div>M.J. Koudstaal</div><div>J.B. Smeets</div><div>G.J. Kleinrensink</div><div>A.J.M. Schulten</div><div>K.G.H. van der Wal</div>
Purpose: This anatomic biomechanical study was undertaken to gain insight into the underlining mechanism of tipping of the maxillary segments during transverse expansion using tooth-borne and bone-borne distraction devices. Materials and Methods: An anatomic biomechanical study was performed on 10 dentate human cadaver heads using tooth-borne and bone-borne distraction devices. Results: The amount of tipping of the maxillary halves was greater in the tooth-borne group, but the difference was not significant. Four of the specimens demonstrated an asymmetrical widening of the maxilla. Conclusions: Segmental tipping was seen in both study groups. In this anatomic model, tooth-borne distraction led to greater segmental tipping compared with bone-borne distraction. Keep in mind, however, that this anatomic model by no means depicts a patient situation, and any extrapolation from it must be done with great care. The fact that the tooth-borne group demonstrated greater tipping might reflect the general opinion that bone-borne distraction causes less segmental angulation than tooth-borne distraction. Some tipping was seen in the bone-borne group, suggesting that overcorrection to counteract relapse will be necessary with this treatment modality. Avoiding moving obstacleshttp://repub.eur.nl/pub/29010/
Mon, 01 Sep 2008 00:00:01 GMT<div>M.P. Aivar</div><div>E. Brenner</div><div>J.B. Smeets</div>
To successfully move our hand to a target, we must consider how to get there without hitting surrounding objects. In a dynamic environment this involves being able to respond quickly when our relationship with surrounding objects changes. People adjust their hand movements with a latency of about 120 ms when the visually perceived position of their hand or of the target suddenly changes. It is not known whether people can react as quickly when the position of an obstacle changes. Here we show that quick responses of the hand to changes in obstacle position are possible, but that these responses are direct reactions to the motion in the surrounding. True adjustments to the changed position of the obstacle appeared at much longer latencies (about 200 ms). This is even so when the possible change is predictable. Apparently, our brain uses certain information exceptionally quickly for guiding our movements, at the expense of not always responding adequately. For reaching a target that changes position, one must at some time move in the same direction as the target did. For avoiding obstacles that change position, moving in the same direction as the obstacle is not always an adequate response, not only because it may be easier to avoid the obstacle by moving the other way, but also because one wants to hit the target after passing the obstacle. Perhaps subjects nevertheless quickly respond in the direction of motion because this helps avoid collisions when pressed for time. Grasping trapezoidal objectshttp://repub.eur.nl/pub/35361/
Sun, 01 Jul 2007 00:00:01 GMT<div>U. Kleinholdermann</div><div>E. Brenner</div><div>V.H. Franz</div><div>J.B. Smeets</div>
When grasping rectangular or circular objects with a precision grip the digits close in on the object in opposite directions. In doing so the digits move perpendicular to the local surface orientation as they approach opposite sides of the object. This perpendicular approach is advantageous for accurately placing the digits. Trapezoidal objects have non-parallel surfaces so that moving the digits in opposite directions would make the digits approach the contact surfaces at an angle that is not 90°. In this study we examined whether this happens, or whether subjects tend to approach trapezoidal objects' surfaces perpendicularly. We used objects of different sizes and with different surface slants. Subjects tended to approach the object's surfaces orthogonally, suggesting that they aim for an optimal precision of digit placement rather than simply closing their hand as it reaches the object. The effects of pause software on the temporal characteristics of computer usehttp://repub.eur.nl/pub/35614/
Thu, 01 Feb 2007 00:00:01 GMT<div>H.P. Slijper</div><div>J.M. Richter</div><div>J.B. Smeets</div><div>M.A. Frens</div>
The study investigated the natural work-pause pattern of computer users and the possible effects of imposing pause regimes on this pattern. Hereto, the precise timing of computer events was recorded across a large number of days. It was found that the distribution of the pause durations was extremely skewed and that pauses with twice the duration are twice less likely to occur. The effects of imposing pause regimes were studied by performing a simulation of commercially available pause software. It was found that depending on the duration of the introduced pause, the software added 25-57% of the pauses taken naturally. Analysis of the timing of the introduced pauses revealed that a large number of spontaneous pauses were taken close to the inserted pause. Considering the disappointing results of studies investigating the effects of introducing (active) pauses during computer work, this study has cast doubt on the usefulness of introducing short duration pauses.Grasping the Müller-Lyer illusion: Not a change in perceived lengthhttp://repub.eur.nl/pub/35635/
Mon, 01 Jan 2007 00:00:01 GMT<div>M. Biegstraaten</div><div>D.D.J. de Grave</div><div>E. Brenner</div><div>J.B. Smeets</div>
Peak grip aperture has often been used to quantify the influence of illusions on judgments of size for action. However, a larger peak grip aperture need not mean that the object looks larger. It could also mean that it was grasped more carefully. These two possibilities can be distinguished on the basis of the velocity of grip closure just before contact. We let people grasp a bar that was placed on the shaft of a Müller-Lyer figure. The Müller-Lyer figure influenced the peak grip aperture. It did not influence the velocity of grip closure in the way that one would expect if size were misperceived. Thus there is no reason to assume that the perceived size guides the way that we reach and grasp an object. Sensory integration does not lead to sensory calibrationhttp://repub.eur.nl/pub/63485/
Tue, 05 Dec 2006 00:00:01 GMT<div>J.B. Smeets</div><div>J.J. van den Dobbelsteen</div><div>D.D.J. de Grave</div><div>R.J. van Beers</div><div>E. Brenner</div>
One generally has the impression that one feels one's hand at the same location as one sees it. However, because our brain deals with possibly conflicting visual and proprioceptive information about hand position by combining it into an optimal estimate of the hand's location, mutual calibration is not necessary to achieve such a coherent percept. Does sensory integration nevertheless entail sensory calibration? We asked subjects to move their hand between visual targets. Blocks of trials without any visual feedback about their hand's position were alternated with blocks with veridical visual feedback. Whenever vision was removed, individual subjects' hands slowly drifted toward the same position to which they had drifted on previous blocks without visual feedback. The time course of the observed drift depended in a predictable manner (assuming optimal sensory combination) on the variable errors in the blocks with and without visual feedback. We conclude that the optimal use of unaligned sensory information, rather than changes within either of the senses or an accumulation of execution errors, is the cause of the frequently observed movement drift. The conclusion that seeing one's hand does not lead to an alignment between vision and proprioception has important consequences for the interpretation of previous work on visuomotor adaptation.Grasping reveals visual misjudgements of shapehttp://repub.eur.nl/pub/56668/
Sun, 01 Oct 2006 00:00:01 GMT<div>R.H. Cuijpers</div><div>E. Brenner</div><div>J.B. Smeets</div>
There are many conditions in which the visually perceived shape of an object differs from its true shape. We here show that one can reveal such errors by studying grasping. Nine subjects were asked to grasp and lift elliptical cylinders that were placed vertically at eye height. We varied the cylinder's aspect ratios, orientations about the vertical axis and distances from the subject. We found that the subjects' grip orientations deviated systematically from the orientations that would give the mechanically optimal grip. That this is largely due to misjudging the cylinder's shape (rather than to selecting a comfortable posture) follows from the fact that the grip aperture was initially more strongly correlated with the maximal grip aperture (which is related to the expected contact positions) than with the final grip aperture (which is determined by the real contact positions). The correlation with the maximal grip aperture drops from 0.8 to 0.6 in the last 1% of the traversed distance (11% of movement time), showing that the grip aperture was anticipated incorrectly (it is automatically "corrected" at contact). The grip orientation was already strongly correlated with the grip orientation at the time of maximal grip aperture, half way through the movement (R≥0.7), showing that the suboptimal grip orientations were planned that way. We conclude that subjects plan their grasps using information that is based on the misperceived shape.Why are saccades influenced by the Brentano illusion?http://repub.eur.nl/pub/62000/
Sun, 01 Oct 2006 00:00:01 GMT<div>D.D.J. de Grave</div><div>J.B. Smeets</div><div>E. Brenner</div>
In the Brentano version of the Müller-Lyer illusion one part looks longer and the other looks shorter than it really is. We asked participants to make saccadic eye movements along these parts of the figure and between positions on the figure and a position outside the illusion. By showing that saccades from outside the figure are not influenced by the illusion, we demonstrate that the reason that saccades along the figure are influenced is that the incorrectly judged length is used to plan the amplitude of the saccade. This finding contradicts several current views on the use of visual information for action. We conclude that actions are influenced by visual illusions, but that such influences are only apparent if the action is guided by the attribute that is fooled by the illusion.The role of uncertainty in the systematic spatial mislocalization of moving objectshttp://repub.eur.nl/pub/67162/
Tue, 01 Aug 2006 00:00:01 GMT<div>E. Brenner</div><div>R.J. van Beers</div><div>G. Rotman</div><div>J.B. Smeets</div>
The relation between force and movement when grasping an object with a precision griphttp://repub.eur.nl/pub/57757/
Mon, 01 May 2006 00:00:01 GMT<div>M. Biegstraaten</div><div>J.B. Smeets</div><div>E. Brenner</div>
When reaching out for objects, the digits' paths curve so that they approach their positions of contact moving more or less perpendicularly to the local surface orientation. This increases the accuracy of positioning the digits and ensures that any forces exerted at contact are nearly perpendicular to the surface, so that friction will prevent the digits from slipping along the surface. When lifting the object a similar force perpendicular to the surface is needed to prevent the object from slipping from one's fingers. In order to determine whether these two issues are dealt with simultaneously we let subjects pick up a cube from three different starting positions and measured the digits' movements and forces from before contact until the moment the cube started moving. The impact force was low. After impact, the digits spent about 200 ms in contact with the surface of the cube before the latter started to move. The digits first decelerated, and then they gradually built up the grip- and lift forces to move the cube upwards. We found no direct relationship between the control of the reaching movement towards the object and the force applied at the surface of the object to pick it up. We conclude that the reaching and lifting movements are quite independent.Two eyes in actionhttp://repub.eur.nl/pub/71239/
Sat, 01 Apr 2006 00:00:01 GMT<div>E. Brenner</div><div>J.B. Smeets</div>
Do relative binocular disparities guide our movements in depth? In order to find out we asked subjects to move a 'cursor' to a target within a simulated horizontal plane at eye height. They did so by moving a computer mouse. We determined how quickly subjects responded to the target jumping in depth. We found that it took subjects about 200 ms to respond to changes in binocular disparity. Subjects responded just as quickly if the cursor was temporarily only visible to one eye near the time that the target jumped in depth, and less vigorously, though just as quickly, if the cursor jumped rather than the target, so the fastest binocular responses cannot be based directly on the relative retinal disparity between the target and the cursor. Subjects reacted faster to changes in the target's height in the visual field than to changes in binocular disparity, but did not react faster to changes in image size. These results suggest that binocular vision mainly improves people's everyday movements by giving them a better sense of the distances of relevant objects, rather than by relative retinal disparities being used to directly guide the movement. We propose that relative disparities only guide parts of very slow movements that require extreme precision.Determining whether a ball will land behind or in front of you: Not just a combination of expansion and angular velocityhttp://repub.eur.nl/pub/61310/
Wed, 01 Feb 2006 00:00:01 GMT<div>A.-M. Brouwer</div><div>J. López-Moliner</div><div>E. Brenner</div><div>J.B. Smeets</div>
We propose and evaluate a source of information that ball catchers may use to determine whether a ball will land behind or in front of them. It combines estimates for the ball's horizontal and vertical speed. These estimates are based, respectively, on the rate of angular expansion and vertical velocity. Our variable could account for ball catchers' data of Oudejans et al. [The effects of baseball experience on movement initiation in catching fly balls. Journal of Sports Sciences, 15, 587-595], but those data could also be explained by the use of angular expansion alone. We therefore conducted additional experiments in which we asked subjects where simulated balls would land under conditions in which both angular expansion and vertical velocity must be combined for obtaining a correct response. Subjects made systematic errors. We found evidence for the use of angular velocity but hardly any indication for the use of angular expansion. Thus, if catchers use a strategy that involves combining vertical and horizontal estimates of the ball's speed, they do not obtain their estimates of the horizontal component from the rate of expansion alone.Hitting moving targets: Effects of target speed and dimensions on movement timehttp://repub.eur.nl/pub/61636/
Mon, 01 Aug 2005 00:00:01 GMT<div>A.-M. Brouwer</div><div>J.B. Smeets</div><div>E. Brenner</div>
To hit moving targets, one not only has to arrive at the right place but also at the right time. Moving quickly reduces spatial precision but increases temporal precision. This may explain why people usually move more quickly toward fast targets than toward slow ones, because arriving at the right time is more important when hitting fast targets. The temporal accuracy required depends not only on the target's speed but also on its length in the direction of motion; it decreases with increasing length. Here we investigate the effects of variations in the target's speed and dimensions on the subject's movement time. We asked subjects to hit targets that moved from left to right as quickly as possible with their index finger. The targets varied in length in the direction of motion (width: affecting both spatial and temporal demands), in length in the orthogonal direction (height: affecting spatial demand), and in speed (affecting temporal demand). Targets were presented in random order during one session and in blocks of trials with identical targets during another session. In the latter session subjects could optimize their strategy for each target separately. In the random condition subjects hit fast targets more quickly than slow ones. Their movement time was also affected by the target's size (the spatial demand), but not by the direction of the elongation. For the blocked condition, subjects did consider the direction of the elongation. We conclude that people do not consider an object's orientation to estimate the temporal demands of an interception task, but that they use the object's size and speed, and their experience from previous trials.Intercepting moving targets: Why the hand's path depends on the target's velocityhttp://repub.eur.nl/pub/57421/
Wed, 20 Jul 2005 00:00:01 GMT<div>E. Brenner</div><div>J.B. Smeets</div>
In order to intercept a moving target one must reach some position at the same moment as the target. Considering that moving towards such a position takes time, it seems obvious that one must determine where one can best intercept the target well in advance. However, experiments on hitting moving targets have shown that the paths that the hand takes when trying to intercept targets that are moving at different velocities are different, even if the targets are hit at the same position. This is particularly evident at high target velocities, which seems strange because the benefit of considering the target's velocity should be largest for fast targets. We here propose that the paths' curvature may intentionally differ for different target velocities in order to maximise the chance of hitting the target. Arriving at the target with a velocity that matches that of the target can reduce the consequence of certain temporal errors. In particular, if the path curves in a way that makes the component of the hand's final velocity that is orthogonal to the hitting direction exactly match the velocity of the target, then no additional error will arise from arriving at the target slightly earlier or later than expected. On the other hand, moving along a curved path is likely to increase the spatial errors. We argue that a compromise between these two influences could account for the differences between paths towards fast and slow targets.Effects of the Ebbinghaus figure on grasping are not only due to misjudged sizehttp://repub.eur.nl/pub/68805/
Sun, 01 May 2005 00:00:01 GMT<div>D.D.J. de Grave</div><div>M. Biegstraaten</div><div>J.B. Smeets</div><div>E. Brenner</div>
It is not evident how the small effects of the flankers of the Ebbinghaus figure on peak grip aperture (PGA) should be interpreted. One interpretation is that the flankers influence the estimated size, which in turn influences the grasp. If this interpretation is correct, then only the size-dependent aspects of the grasping movement should depend on the spatial positions of the flankers. An alternative interpretation is that the effect on grip aperture is caused by a change in judgement of the required precision, in which case various aspects of the grasping movement could be influenced by the size and position of the flankers. We presented subjects with a display consisting of a central disk surrounded by four large or small flankers. The array of circular flankers could be rotated by 45°. There were two tasks: to reproduce the perceived size of the central disk, and to grasp the central disk. As in other studies, the reproduced size and the PGA were both influenced by the size of the flankers. The effect on reproduced size settings was independent of the flankers' spatial position. Nevertheless, the flankers' position did influence the final grip aperture and the grip orientation at PGA and at movement offset. Because the flankers changed more than only the PGA, we conclude that the effect of the flankers on prehension cannot only be because of misjudgement of the size of the central disk.Flashes are localised as if they were moving with the eyeshttp://repub.eur.nl/pub/59627/
Tue, 01 Feb 2005 00:00:01 GMT<div>G. Rotman</div><div>E. Brenner</div><div>J.B. Smeets</div>
Targets that are flashed during smooth pursuit are mislocalised in the direction of the pursuit. It has been suggested that a similar mislocalisation of moving targets could help to overcome processing delays when hitting moving objects. But are moving targets really mislocalised in the way that flashed ones are? To find out we asked people to indicate where targets that were visible for different periods of time had appeared. The targets appeared while the subjects' eyes were moving, and were either moving with the eyes or static. For flashed targets we found the usual systematic mislocalisation. For targets that moved with the eyes the mislocalisation was at least as large, irrespective of the presentation time. For static targets the mislocalisation decreased with increasing presentation time, so that by the time the presentations reached about 200 ms the targets were not mislocalised at all. A simple model that combines smooth retinal motion with information about the velocity of smooth pursuit could account for the measured tapping errors. These findings support the notion that the systematic mislocalisation of flashed targets is related to the way in which people intercept moving objects.Luminance-color correlation is not used to estimate the color of the illuminationhttp://repub.eur.nl/pub/74600/
Tue, 25 Jan 2005 00:00:01 GMT<div>J.J.M. Granzier</div><div>E. Brenner</div><div>F.W. Cornelissen</div><div>J.B. Smeets</div>
Humans can identify the colors of objects fairly consistently, despite considerable variations in the spectral composition of the illumination. It has been suggested that the correlation between luminance and color within a scene helps to disentangle the influences of illumination and reflectance, because the surfaces that reflect the light of the illuminant well will normally be bright. Because the reliability of the luminance-color correlation as an indicator of the chromaticity of the illuminant depends on the number of surfaces that are considered, we expected the correlation to be determined across large parts of the scene. To examine whether this is so, we compared different scenes with matched luminance and chromaticity, but in which the correlation between luminance and chromaticity was manipulated locally. Our results confirm that there is a bias in perceived color away from the chromaticity of bright surfaces. However, the results show that only the correlation within about 1° of the target is relevant. Thus, it is unlikely that the visual system uses the correlation between luminance and color to explicity determine the chromaticity of the illuminant. Instead, this correlation is presumably implicitly considered in the way that the color contrast at borders is determined.Components of motion perception revealed: Two different after-effects from a single moving objecthttp://repub.eur.nl/pub/62195/
Fri, 01 Oct 2004 00:00:01 GMT<div>J. López-Moliner</div><div>J.B. Smeets</div><div>E. Brenner</div>
If motion that one has been looking at for some time suddenly stops, or if one shifts one's gaze to a static object, one will see motion in the opposite direction: the motion after-effect. If two transparent surfaces move with different speeds in different directions, then the direction of the motion after-effect will depend on the test pattern. For such transparent surfaces both the local motion and the global percept have two components. When looking at a normal moving object, there is only one perceived global motion. However, we know that locally there can be considerable ambiguity (the aperture problem). Does one adapt to all the local components, including those that one does not perceive, or only to the perceived global motion? We designed a stimulus that is perceived to be a fast rotating object, but also has a slow local radial component of motion. By selecting an appropriate test pattern we could either get a radial or a rotating motion after-effect. Thus we show that adaptation to motion must (also) occur at a stage at which local motions have not yet been integrated to give a unified percept.Colour vision can contribute to fast corrections of arm movementshttp://repub.eur.nl/pub/69462/
Fri, 01 Oct 2004 00:00:01 GMT<div>E. Brenner</div><div>J.B. Smeets</div>